Hi,
if you were to ovulate on cd 20 and only have a 28 day cycle that would not be enough time for the egg to properly implant itself and your lining would be starting to break up, I guess would be a way to word it...
It should be at least 10 days I have read so that there is enought time. Not saying it is impossible, someone could implant real quick too, like 4-5 days.
You can't really test for it by yourself unless you do a bbt chart for a few months in a row. They are effective, I was messed up in my cycles and in stead of having to do 100 tests and wait forever my dr was able to look at my chart and see what was going on.